How far are you through the game? I heard reports that at some points in the game you can't go to Liberty Island. I think it's because the Team Plasma characters are tied up elsewhere, e.g. Dragonspiral Tower, so they can't be in two places at once.
So the solution would be to finish any current quests you are on like Dragonspiral Tower, and then you should be able to go back and visit Liberty Island.